Within the electronics industry, there is a trend towards automated inspection of a circuit board following placement of components on it to detect if any of the components have been improperly placed or are missing. Further, efforts are now underway to inspect the circuit board before component placement but immediately after application of a layer of solder paste, which is applied to bond the components, to detect if the paste has been properly applied. By inspecting the circuit board at each stage of its fabrication, the board can be repaired to correct such defects more economically. For example, if the solder paste has been applied improperly, it is far easier, and hence less expensive, to clean the board and reapply paste to it prior to placement of the components rather than afterwards. Similarly, missing and misplaced components can be corrected more easily if such defects are detected prior to reflow of the solder paste to solder bond the components to the circuit board rather than after reflow of the paste.
In my co-pending application "Three-Dimensional Imaging Using Sharp Gradient of Illumination", Ser. No. 440,948, filed on Nov. 24, 1989, and assigned to AT&T (herein incorporated by reference), there is disclosed a three-dimensional imaging system especially suited for circuit board inspection. The imaging system described in my co-pending application comprises a linescan camera positioned so its optical axis is generally perpendicular to the surface of a circuit board to capture the image of a single strip of area on the board surface. Each of a pair of light sources is separately energized to generate a line of light directed at the surface of the circuit board from the same side, at substantially the same angle, to illuminate a strip of area on the board surface. While each light source is energized, a relative motion is imparted between the linescan camera and the circuit board so that the camera senses the intensity of the light reflected from successive strips of area on the board surface. The ratio of the reflectance intensities measured when the circuit board is illuminated with each of the two light sources yields a measure of the height of the topographical features within each successive strip. From the height information, a three-dimensional image of the circuit board can be obtained.
While the above-described technique enables a three-dimensional image of a substrate to be obtained in a very efficient manner, the technique, as with other similar types of triangulation techniques for three-dimensional imaging, does incur a drawback which limits its effectiveness. When a relatively tall topographical feature, such as a component, is situated in close proximity to a short feature, such as a component or solder paste deposit, the taller feature may occlude (shadow) the shorter one depending on the direction of the light. As a consequence, an accurate measurement of the height of the shorter feature may be difficult to ascertain, thus adversely affecting the resultant three-dimensional image of the circuit board.
Thus, there is a need for a technique for obtaining a three-dimensional image of a substrate with reduced incidence of occlusion.